Determination of k Q factors for cylindrical and plane-parallel ionization chambers in a scanned carbon ion beam by means of cross calibration

Monte Carlo-calculated beam quality and perturbation correction factors validated against experiments for Farmer and Markus type ionization chambers in therapeutic carbon-ion beams

Objective. In current dosimetry protocols, the estimated uncertainty of the measured absorbed dose to waterDwin carbon-ion beams is approximately 3%. This large uncertainty is mainly contributed by the standard uncertainty of the beam quality correction factorkQ. In this study, thekQvalues in four cylindrical chambers and two plane-parallel chambers were calculated using Monte Carlo (MC) simulations in the plateau region. The chamber-specific perturbation correction factorPof each chamber was also determined through MC simulations.Approach.kQfor each chamber was calculated using MC code Geant4. The simulatedkQratios in subjected chambers and reference chambers were validated through comparisons against our measured values. In the measurements in Heavy-Ion Medical Accelerator in Chiba,kQratios were obtained fromDwvalues of60Co, 290- and 400 MeV u-1carbon-ion beams that were measured with the subjected ionization chamber and the reference chamber. In the simulations,fQ(the product of the water-to-air stopping power ratio andP) was acquired fromDwand the absorbed dose to air calculated in the sensitive volume of each chamber.kQvalues were then calculated from the simulatedfQand the literature-extractedWairand compared with previous publications.Main results. The calculatedkQratios in the subjected chambers to the reference chamber agreed well with the measuredkQratios. ThekQuncertainty was reduced from the current recommendation of approximately 3% to 1.7%. ThePvalues were close to unity in the cylindrical chambers and nearly 1% above unity in the plane-parallel chambers.Significance. ThekQvalues of carbon-ion beams were accurately calculated in MC simulations and thekQratios were validated through ionization chamber measurements. The results indicate a need for updating the current recommendations, which assume a constantPof unity in carbon-ion beams, to recommendations that consider chamber-induced differences.

Direct determination of k Q for Farmer-type ionization chambers in a clinical scanned carbon-ion beam using water calorimetry

Within two studies, k Q factors for two Farmer-type ionization chambers have been experimentally determined by means of water calorimetry in the entrance channel (EC) of a monoenergetic carbon-ion beam (Osinga-Blättermann et al 2017 Phys. Med. Biol. 62 2033–54) and for a passively modulated spread-out Bragg peak (SOBP) (Holm et al 2021 Phys. Med. Biol. 66 145012). Both studies were performed at the Heidelberg Ion Beam Therapy Center (HIT) using the PTB portable water calorimeter but applying different initial beam energies of 429 MeV u−1 for the EC and 278 MeV u−1 for the SOBP as well as different scanning patterns of the irradiated field. Comparing their results revealed differences between the experimental k Q factors of up to 1.9% between the EC and the SOBP. To further investigate this unexpected difference, we performed additional k Q determinations for the EC of an 278 MeV u−1 monoenergetic carbon-ion beam and reevaluated the original data of Osinga-Blättermann et al (2017 Phys. Med. Biol. 62 2033–54). This new experimental data indicated no difference between the k Q factors for the EC and the SOBP and the reevaluation led to a substantial reduction of the originally published k Q factors for the EC of the 429 MeV u−1 beam (Osinga-Blättermann et al 2017 Phys. Med. Biol. 62 2033–54). Finally, no significant difference between the data for the EC and the data for the SOBP can be found within the standard measurement uncertainty of experimental k Q factors of 0.8%. The results presented here are intended to correct and replace the k Q data published by Osinga-Blättermann et al (2017 Phys. Med. Biol. 62 2033–54) and in Osinga-Blättermann and Krauss (2018 Phys. Med. Biol. 64 015009).

Monte Carlo-derived ionization chamber correction factors in therapeutic carbon ion beams

The accuracy of electromagnetic transport in the GEANT4 Monte Carlo (MC) code was investigated for carbon ion beams and ionization chamber (IC)-specific beam quality correction factors were calculated. This work implemented a Fano cavity test for carbon ion beams in the 100-450 MeV/u energy range to assess the accuracy of the default electromagnetic physics parameters. TheUrbanand theWentzel-VImultiple Coulomb scattering models were evaluated and the impact ofmaxStep,dRover,andfinal rangeparameters on the accuracy of the transport algorithm was investigated. The optimal production thresholds for an accurate calculation offQvalues, which is the product of the water-to-air stopping power ratio and the IC-specific perturbation correction factor, were also studied. ThefQcorrection factors were calculated for a cylindrical and a parallel-plate IC using carbon ions in the 150-450 MeV/u energy range. Modifying the default electromagnetic physics parameters resulted in a maximum deviation from theory of 0.3%. Therefore, the default EM parameters were used for the remainder of this work. ThefQfactors were found to converge for both ICs with decreasing production threshold distance below 5μm. ThefQvalues obtained in this work agreed with the TRS-398 stopping power ratios and other previously reported results within uncertainty. This study highlights an accurate MC-based technique to calculate the combined stopping power ratio and the perturbation correction factor for any IC in carbon ion beams.

Water calorimetry-basedkQfactors for Farmer-type ionization chambers in the SOBP of a carbon-ion beam

The dosimetry of carbon-ion beams based on calibrated ionization chambers (ICs) still shows a significantly higher uncertainty compared to high-energy photon beams, a fact influenced mainly by the uncertainty of the correction factor for the beam qualityk_Q. Due to a lack of experimental data,k_Qfactors in carbon-ion beams used today are based on theoretical calculations whose standard uncertainty is three times higher than that of photon beams. To reduce their uncertainty, in this work,k_Qfactors for two ICs were determined experimentally by means of water calorimetry for the spread-out Bragg peak of a carbon-ion beam, these factors are presented here for the first time. To this end, the absorbed dose to water in the¹²C-SOBP is measured using the water calorimeter developed at Physikalisch-Technische Bundesanstalt, allowing a direct calibration of the ICs used (PTW 30013 and IBA FC65G) and thereby an experimental determination of the chamber-specifick_Qfactors. Based on a detailed characterization of the irradiation field, correction factors for several effects that influence calorimetric and ionometric measurements were determined. Their contribution to an overall uncertainty budget of the finalk_Qfactors was determined, leading to a standard uncertainty fork_Qof 0.69%, which means a reduction by a factor of three compared to the theoretically calculated values. The experimentally determined values were expressed in accordance with TRS-398 and DIN 6801-1 and compared to the values given there. A maximum deviation of 2.3% was found between the experiment and the literature.